It is known that biological compounds such as amino acids and sugars often have optical isomers (also referred to as enantiomers). It is known that optical isomers are generally distinguished as the D-form and the L-form, and the properties and actions are different between the D-form and the L-form in vivo. As disclosed in Patent Literature 1, for example, an increase in an optical isomer of a specific compound may be used as a marker of a specific disease. In addition, some reports point out that an increase in an optical isomer of a specific compound may cause a specific disease. Therefore, in the fields of medicine, pharmaceutical development, food, and the like, there is a need for a method by which optical isomers (D-form, L-form) of a compound are easily distinguished and by which the ratio of the optical isomers (optical purity) is easily calculated.
Conventionally, liquid chromatography is used to separate optical isomers in a sample. In addition, in order to detect or distinguish the separated optical isomers, optical measurement such as circular dichroism (CD) measurement or optical rotation measurement based on a light absorption spectrum is used. However, generally, analysis by such methods takes time. In addition, in order to use such methods, an adequate amount of sample is necessary, and it is difficult to use the methods when the amount of sample is very small.
Apart from the above analytical methods, a method for analyzing optical isomers using ion mobility spectrometry (IMS) is conventionally known. When molecular ions generated from a compound contained in a sample are moved in a medium gas (or liquid) by the action of an electric field, the ions move at a speed corresponding to mobility that depends on a collision cross-section depending on the size or the like of the molecule and that depends on the strength of the electric field. The IMS method is a measurement method using this mobility for analysis of sample molecules (see Patent Literature 2, for example).
The structures of the D-form and the L-form, which are optical isomers, are in a mirror image relation and have the same mass and size, so that there is no difference in a collision cross-section. Therefore, optical isomers cannot be separated by the general IMS method. Non Patent Literatures 1 and 2 disclose the following special analytical method. In this method, a chiral gas having a structure similar to the structure of a compound to be detected is mixed into a gas flowing in a region where ions are drifted in the IMS method. Then, an interaction that depends on chiral symmetry occurs between the compound molecular ions to be detected and the chiral gas, and the effective collision cross-section of the compound molecule ions to be detected changes. By using this characteristic, optical isomers can be separated and detected in the IMS method.
However, in the above method, it is necessary to prepare a chiral gas corresponding to the compound to be detected. Therefore, the method works well only when the compound is to be detected is a specific one or ones. If various compounds are to be detected, however, it is necessary to prepare various types of chiral gases corresponding to each of these compounds, and the measurement cost is accordingly high. In addition, for some kinds of compounds, an appropriate chiral gas cannot be prepared, and in such a case, the above analysis method cannot be used.